Auxin And Gibberellins Involved In The Changes Of Maize Root Morphology During Phosphorous Starvation

The root system architecture (RSA) of terrestrial plants is involved in anchorage of the plant, as well as the acquisition of water and nutrients. Phosphate (Pi) plays a important role during plant growth and development. Diffusion rates of Pi in the soil solution are generally very low because soil particles easily bind Pi. Therefore, RSA is very important for plant Pi acquisition. Phytohormones are crucial elements for plant growth and development, which also determine the root organization. Among them auxin and gibberellins show to boost plant growth, but their roles in plants suffered from phosphate starvation has not been understood.In this paper the seedlings of maize inbred line Qi-319 were used to indicate the changes of root morphology under phosphate deficiency conditions. As previous results of our lab, axile roots were elongated and lateral roots density and length were reduced under phosphate deficiency conditions for 8 days. The lateral root density and length were promoted by 0.01μmol/L NAA, while the length of axile roots (LAR) was not influenced under sufficient phosphate conditions. When maize plants grew under phosphate deficiency conditions, the production of lateral roots was greatly induced by the NAA treatment, and the lateral root density and length were higher than those of untreated plants. The axile root growth and lateral root emergence were inhibited when applying 0.1μmol/L N-1-naphthylphthalamic acid (NPA), a polar auxin transport inhibitor, and the lateral root growth was also inhibited under phosphate deficiency conditions.The length of axile and lateral roots of maize was induced by the treatment with 0.1μmol/L GA3, but the lateral root density was reduced. When maize grew under deficient phosphorus conditions,0.1μmol/L GA3 increased the length of lateral roots and decreased the LAR, and had no effects on lateral root production. PP333, which inhibits GAs synthesis, significantly inhibited the LAR, and no obvious effects on the lateral root density and length. These results implied that auxin and gibberellins displayed positive roles to the root growth of maize under sufficient phosphate conditions, and also participated in the modification of the root system architecture under phosphate deficiency conditions.Expression of some genes that could be involved in the changes of root system architecture in maize root tip and section of lateral root primordium formation was examined by real time RT-PCR to determine which ways auxin and gibberellins participated in to modify the root system architecture. The genes included ZmARF1～ARF6, ZmARF10, ZmTIR1, ZmAUX1, ZmPIN1a and ZmPIN1b, ZmPP2C, ZmPP2AA1, ZmPP2AA2, ZmPID, ZmAN1and ZmAN2, ZmEKO, ZmGA2ox1, ZmGA2ox2, ZmGA20ox1, ZmGA20ox2, ZmGID1a, ZmGID1b, ZmDELLA, ZmEXP1, ZmEXPB1～B4, ZmVP1-1, ZmVP1-2, ZmVP2, ZmKRP1-3, ZmIPS1, ZmMDGD, ZmPhtl; 1, ZmSDQl, ZmLPR1, which involved in auxin response pathway and auxin polar transport, GA synthesis pathway and GA response pathway, root system development and phosphate starvation response, respectively.In auxin response pathway the transcript abundance of genes in ARF family showed obvious difference under sufficient phosphate conditions, and the ARF1, ARF5 and ARF 10 showed higher transcript abundance under phosphate deficiency conditions while ARF4 had reduced expression level. Moreover, the expression level of TIR1 went up by the induction of phosphate deficit. When treated the plant with NAA under sufficient phosphate conditions, the expression levels of members in ARF family and TIR1 went up at first, then decreased, and there were great differences on the expression levels among the members. When maize grew under phosphate deficiency condition, the effects of NAA on the root morphology became weak.The expression level of ZmAUX1, ZmPIN1a and ZmPIN1b, ZmPP2AA1 and ZmPP2AA2 in root tip and section of lateral root primordium formation showed distinct diversity, and the transcript abundance was regulated by NAA treatment. Under phosphate deficiency conditions, the effects of NAA on the expression of the genes grew weak. Moreover, the expression of KRP1 in section of lateral root primordium formation was decreased by NAA treatment, and the expression level of the gene did not show remarkable changes under NAA and phosphate deficiency condition. Those results implied that NAA could take part in the development of lateral roots and the modification of RSA via the regulation of expression of genes which involved in polar auxin transport and distribution.Under phosphate deficiency conditions, the expression levels of many genes involved in GA synthesis had significant changes, which was beneficial to lead the accumulation of active GAs. The transcript abundance of GID1a and GID1b coded gibberellin receptor substantially increased under phosphate deficiency conditions, which meant the flow quantity of GA signal was enlarged. The treatment of GA3 or both GA3 and phosphate deficit decreased the expression levels of some genes that controlled earlier steps in the process of GA synthesis, and the expression levels of GA20ox1 and GA20ox2 in root tip and section of lateral root primordium formation significantly decreased, while the expression levels of GA2ox1 and GA2ox2 were up-regulated. It was inferred that the effects of GA3 treatment on the gene expression were opposing to the effects of phosphate deficit by the examining the expression levels of GA20ox1, GA20ox2, GA2ox1 and GA2ox2 after GA3 treatment. The transcript abundance of KRP2 in root tip was enhanced, while the expression level of KRP2 in root tip and section of lateral root primordium formation was reduced. These results suggested that the gibberellin took part in the regulation of cell elongation and division.The expression of IPS and phosphate transporter gene Pht1;1 in the section of lateral root primordium formation was promoted by GA3 treatment, and the transcript abundance greatly increased under phosphate deficiency conditions, but no effects on the phosphorus concentrations in plants, which meant that GAs regulated the reaction of maize to the phosphate deficiency conditions. The expression of a few of genes in EXP family was enhanced by the phosphate deficiency conditions, which implied that the cell wall could be reconstructed by the higher activity of expansion after GA3 treatment, and promoted the cell expansion and the alteration of root system architecture.